ASP.NET Core Model Validation: DataAnnotations and ModelState

Model validation helps prevent invalid input from reaching application logic. ASP.NET Core can validate incoming models using attributes and other validation mechanisms.

🏷️ DataAnnotations

public class Product
{
    [Required]
    public string Name { get; set; } = "";

    [Range(1, 100000)]
    public decimal Price { get; set; }
}

The attributes describe basic validation rules close to the model.

🌐 MVC Validation

[HttpPost]
public IActionResult Create(Product model)
{
    if (!ModelState.IsValid)
        return View(model);

    // Save the valid model.
    return RedirectToAction("Index");
}

When validation fails, ModelState.IsValid becomes false and the controller can return the form with validation messages.

🚀 API Validation

With controller-based APIs using [ApiController], ASP.NET Core can automatically return a validation error response when the incoming model is invalid.

🧠 Why Validate at the Boundary?

Reject malformed input early.
Keep business logic focused on valid data.
Provide useful feedback to clients.
Reduce repeated validation code.

🔧 Custom Validation

When an attribute is not enough, you can implement custom validation logic or use a dedicated validation layer for more complex business rules.

🧠 Key Takeaway

Validate incoming data at the application boundary, then let the rest of your code work with trusted, validated values.

Continue with ⚡ ASP.NET Core Tutorials.

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